in order to ascertain the height of Mountains .■ 573 
2d, If then the weight of the whole atmofphere at 
each place can be afcertained, the weight of this column, 
viz. their difference, will be known. 
3d, But the height of the quickfilver in the barome- 
ter expreffes the total weight of the atmofphere in the 
place of obfervation; the difference, therefore, of the 
height of the barometer, obferved in two places at the 
fame time, willexprefs the. difference of elevation of the 
two places. 
4th, But further, the weight of this column of the 
atmofphere is liable to fome variations, being diminifhed 
by heat, and augmented by cold; and again, a fimilar 
alteration takes place in the column of quickfilver, 
which is the meafure of this weight. 
5th, If then the degree of thefe variations can be de- 
termined, and the temperature of the air and quickfilver 
at the time of obfervation be known, the weight of this 
column of air, or the difference of elevation of the two 
places, will be concluded as certainly as if the gravity of 
thefe two fluids, with all heats, remained invariably the 
fame: this is the whole myftery of barometrical mea- 
furement. 
A 
